In a typical fluid filtration application, cylindrical depth filters are housed within pressure vessels. During the filtration process the vessel is pressurized to force the fluid through the depth filter media. As the fluid flows through the filter, particles are trapped by the fibers that make up the depth filter media. The fluid that has been filtered (the “filtrate”) is then forced from the filter housing by the pressure within the vessel.
FIG. 23 illustrates a known filtration device consisting of a cylindrical filter cartridge 231 containing a cylindrical case 233 which has an inlet 234 and an outlet 235. The fluid being filtered enters the inlet and flows through the filter 231, which removes and retains larger particles passing through openings of the filter, but allows the carrier fluid (the filtrate) to pass through and exit via the outlet 235. The effective filtering area of this device is larger than the inlet area. Thus, the speed of flow through the filter is slower than that in the inlet, and the direction of the flow through the filter 231 is perpendicular to the surface of the filter. This situation results in formation of a cake layer blocking openings of the filter, whereby the effective filtration area is reduced. When the effective area of the filter becomes smaller than the inlet area, the effective pressure (ΔP) between inlet 234 and outlet 235 increases. For typical applications, there is a maximum pressure drop allowed for the filtration device. FIG. 24 shows the relationship of operation time against pressure drop across filtration devices of the prior art. At the beginning of a filtration operation, the pressure drop is generally constant as long as the effective filtration area is larger than the cross sectional area of the inlet 234. The effective filtration area will gradually be reduced due to formation of a cake layer on the filter. When the effective filtration area becomes smaller than the cross sectional area of the inlet 234, the pressure drop across the filter device will suddenly increase, and quickly exceed the maximum value of the device.
As the cylindrical filter cartridge 231 becomes clogged with particles, the overall performance of the filter system decreases. To a minimal extent, cartridge filters can be cleaned by mechanical and/or fluid agitation. As a result, one significant limitation of a cartridge filter is a relatively short usable life, particularly in applications having high particle counts.
A need exists for a filtration system which can minimize the effects of pressure drop across a filter media. This is also a need for a self-cleaning filter assembly providing improved surface cleaning capabilities.